The largest fire in modern California history burned in December, and the Thomas Fire is still under investigation. A growing number of lawsuits claim a failed water system is a key reason hundreds of homes burned in the city of Ventura. Stephanie O'Neill reports.

STEPHANIE O'NEILL, BYLINE: While fire hydrants in most cities are connected to a central water system, some are not. Many hillside neighborhoods in California, for instance, get their supply from hilltop tanks that get pumped full at night and then provide water through the day. And that's how the water system works in Ventura, Calif., where resident Rick Ray and his wife, Sharon, live at the top of a foothill with sweeping ocean views.

RICK RAY: If you look up here, you'll see we have two giant water tanks here. These are city water tanks, 250,000 gallons each. When I moved in here, I thought, boy, what safer place to live than right below these giant water tanks?

O'NEILL: On the evening of December 4, Ray got word of the wind-whipped wildfire heading his way. For several hours, he protected his house with help from a rooftop sprinkler system he'd installed for exactly this situation.

RAY: I had water rolling over the eaves of the roof here, keeping the entire property wet.

O'NEILL: Assuming the water tanks were full, Ray planned to run the sprinklers all night. But around 10 p.m., the fire caused a power outage throughout Ventura County and beyond. Without power, the pumps can't push water uphill to refill the tanks, and a few hours later, Ray's sprinklers stopped.

RAY: In the middle of the firefight, right - really right at the height of the firefight, all water up here on the hillside quit. It wasn't just in this neighborhood. It was all across the foothills that water began failing. At that point, all I could do was run around and stamp on the ashes with my feet.

O'NEILL: Ultimately, the Thomas Fire destroyed more than 500 homes in Ventura and damaged nearly 150 others, according to state fire officials. And that's left residents asking what happened. Ventura City officials aren't offering many answers, in part because of the lawsuits against them. But Ventura Water general manager Kevin Brown did offer a prepared statement.

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KEVIN BROWN: The system lost pressure because water was being drained at a tremendous rate due to the extent of the fire response and due to leaks caused by fire damage.

O'NEILL: With Ventura officials refusing to respond to NPR's questions, I turned to nearby Los Angeles for some answers. About a quarter of LA's water supply is pump-driven into hillside tanks. LA typically refills those when they fall below a certain level. But when the National Weather Service issues red flag warnings for extreme fire danger, they start filling the tanks immediately. Richard Harasick is head of water systems for the LA Department of Water and Power.

RICHARD HARASICK: What we do in a red flag warning is we make sure that all those tanks are topped off, meaning they're full, so that if we do lose power because of a fire that we have enough water to fight those fires within those tanks.

O'NEILL: And when wildfires kill the electricity, LA relies on automated backup systems to fill the tanks.

HARASICK: Many of those pumping stations will have pumps that can run off a internal combustion engine that's located right there at the pumping station.

O'NEILL: The city of Ventura, by contrast, has no permanent or automatic backup power at its pumping stations. That's according to Joseph Richardson, a former supervisor with Ventura Water. Instead, he says, the city uses portable generators that take time to deploy.

JOSEPH RICHARDSON: I knew those people were in trouble. With the power outage, they weren't going to have pump station capability to meet the water demands up there.

O'NEILL: Whether Ventura takes any preventive actions during red flag warnings they won't say. What they will say is that they constantly evaluate their water system, that they'll review how it performed during the Thomas Fire and will make changes as necessary. For NPR News, I'm Stephanie O'Neill in Ventura, Calif. Transcript provided by NPR, Copyright NPR.